Good-bye air bubbles

21st April 2016
Posted By : Nat Bowers
Good-bye air bubbles

When manufacturing electronic components it is important to protect the workpieces against future external influences. Most manufacturers use potting or filling to do so. Especially for high-performance applications, vacuum dispensing is the method of choice. A high number of manufacturers are already using this highly efficient type of dispensing.

Others have not tried it yet, because they consider it too complex or shy away from the supposedly high up-front investment. A compilation of best-practice tips will show that vacuum dispensing is not rocket science. We demonstrated the evident advantages in a comparison test described in this article. Perhaps now even the most sceptical engineer will want to give this method a try at last.

Automotive, industrial or consumer electronics: in all applications electronic components such as chips or entire PCBs must be protected against mechanical or chemical stress. One typical example are motor control sensors such as exhaust sensors or components that get in contact with hot gearbox oil. Other applications involve unprotected wire coils and the enamelled copper wires of wire coiled components. Depending on their usage, any one of these components may be exposed to considerable stress in terms of corrosion, vibrations, moisture or high voltage. Since injection molding is a highly complex production technique and production costs of the molds are considerable, most manufacturers choose potting or filling for their production environment.

Conventional metering and dispensing methods, however, often do not cover all requirements. For example, the tiny gaps between coiled wires may trap bubbles of air, which break the insulating cover and diminish or even ruin high-voltage resistivity. Air bubbles caught below PCBs expand when heated - by several mm if the geometry so allows - which causes tensile loads to act on the coating and on the PCB. In this case, high voltage surges may rupture the coating even if flexible and voltage resistant adhesives are used. Aggressive chemicals such as oil may get in contact with the unprotected surface and damage the component.

Pot-Coil

To prevent air from being trapped between component and coating, parts with complex geometries should be processed under vacuum during dispensing. (image: Scheugenpflug)

The solution for demanding cases

Vacuum dispensing is the method of choice especially in the production of high-performance electronic assemblies. This dispensing method - also suitable for in-line integration - produces a vacuum of 1mbar or less if required, which prevents air from getting caught between component and coating. Vacuum dispensing is not only ideal for expensive high-voltage parts and safety-related components, it also handles assemblies with complex geometries, undercuts or extremely narrow gaps. Besides the purely functional aspects vacuum dispensing may also be an interesting method from a design point of view. For example, the often complex shapes of flared side skirts or the production of illuminated buttons and switches put higher demands on the production technology used. As these parts are always in direct view of their users even the tiniest inhomogeneities and air bubbles in these design pieces and components will render them rejects. Vacuum dispensing in these cases allows results that yield high-quality appearance and texture - an advantage that other dispensing methods only achieve at very high costs if at all.

From a technical point of view the generation of a perfect vacuum, which is entirely void of air, is not required. Vacuum in this context means the reduction in pressure down to approximately 1mbar. The further the air pressure is reduced, the longer the process takes and the higher are the energy costs involved. Also, not every component can sustain a strong pressure reduction, a fact to bear in mind when applying a vacuum. While wire coiled components are mostly insensitive to atmospheric pressure, air encapsulated within a capacitor can cause the component to burst when exposed to an external vacuum. Therefore, the vacuum level should always be matched to the task at hand.

To guarantee that no bubbles are introduced, the complete preparation, feeding and metering process must be carried out in a vacuum. Then, a process called thin-film degassing performed by a high-end material preparation and processing system removes all traces of dissolved air. An agitator is used to further speed up the degassing process by stirring and circulating the dispensing material. This lets all the contained air rise to the surface of the material where it gets in contact with the surrounding vacuum. The degassing effect occurs at the surface layers of the material. In order to prevent air from re-entering into the material during material reloading all fittings, material feed lines, pumps and valves are sealed air-tight.

Vacuum chamber

To guarantee efficient potting and bonding it is essential to choose the correct vacuum value and the right duration of the evacuation for a specific workpiece and for the entire process. This image shows: evacuation of the vacuum chamber. (image: Scheugenpflug)

The agitator and a well-timed circulation of the material in tanks, pumps and feed lines keep the dispensing medium homogeneous at all times. This prevents sedimentation of the contained filler materials, which can easily occur during production breaks. For practical purposes it proved helpful to design dispensing systems specifically for abrasive media. These systems allow the processing of dispensing materials that contain hard and abrasive fillers. As they are purpose-built for the task, they run efficiently at low maintenance and servicing costs.

Comparison test: vacuum dispensing vs. atmospheric-pressure dispensing

We wanted to demonstrate the difference between vacuum and atmospheric-pressure dispensing to manufacturers who still hesitate to try and use vacuum dispensing systems. To do so, DELO Industrie Klebstoffe and Scheugenpflug decided to set up a test system made of standard equipment. We used a conventional PCB in a standard PBT plastic housing as the substrate and a low viscosity two-component acid anhydride epoxy resin as the potting resin to simulate a common automotive application. The resin is specifically designed for high-performance applications and provides long lasting temperature stability up to +200°C. It is resistant against diesel, gasoline and oil, and protects motor and exhaust control sensors during engine operation.

Vacuum casting

Potted standard components: transparent version for the purpose of illustration, black in typical applications. (image: DELO)

The test potting under ambient air conditions was carried out with the two components manually mixed and metered. For the vacuum test run the dispensing material additionally was degassed and then metered and dispensed. To obtain a laboratory-scale setup we used the Scheugenpflug LeanVDS system, a typical entry-level model for vacuum dispensing. This compact-sized system is best suited for R&D applications, for small batch production and to replace inaccurate or time-consuming auxiliary processes such as post-evacuation.

In order to analyse the test results DELO and Scheugenpflug thought it best to use x-ray scans. As opposed to microsections this test method is non-destructive and has the additional benefit of making sure that no air bubbles are overlooked if the section was made at a position that just happened to be free from bubbles. When comparing the x-ray images of the two components distinct differences immediately became evident. While the PCB on the left was potted bubble-free under vacuum, a large bubble of air had formed under the PCB on the right, which was potted under ambient air conditions. What does that mean? Depending on how the component is used later and on its operating environment the component is very likely to fail - even if this might take several months or sometimes even years.

Casted components

X-ray imaging allows non-destructive verification whether potting was completely free of air bubbles. While the PCB on the top was potted bubble-free under vacuum, a large air bubble formed under the PCB on the bottom, which was potted under ambient air conditions. (image: DELO)

Conclusion

Depending on the dispensing system used bubbles can easily form already during preparation, feeding and delivery, or while mixing one- or two-component dispensing masses. Large inclusions of air prove even more fatal to the reliable functioning of electronic assemblies or the visual high-quality appearance of exclusively shaped design parts. These tend to occur near undercuts or in components where production conditions require them to have complex geometries. By combining the right dispensing material with an all-in-one vacuum dispensing, preparation and feeding system manufacturers have all the instruments they need to increase their components' reliability to an extent where they comply with all necessary thermal, mechanical, chemical and design requirements.


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